If I play {Deserter's Quarters} and use mana to tap one of my oppenents creatures. On my upkeep I untap it and use it again on another creature. During his upkeep do both creatures remain tapped or just the last one?
They both remain tapped until his next untap step, at which point the second one stays tapped and the first one untaps.
In other words, just the last one.
Yep. Now that I'm rereading my answer it seems a bit complicated. Pleeb's is better :P
Mine is easier to read, but yours is more complete